[spoiler]I have zero inside information on this, but while it's more satisfying to watch 1995 and Summer? 1970 without knowing they're the set-up for 1840, I'm sure... everything in them is about 1840. They're doing little but preparing us for 1840 from the moment 1995 starts.
I picture DC or the team deciding that they can't think of more films or books to rewrite, so they're going to organize the whole forseeable remainder of the show into a very long storyline where they rob ideas from
themselves, taking everything that worked really well earlier on DS, and doing it bigger and better (and longer). The story of DS is in a way wrapped up at the end of it all, a happy ending tied in a bow, so I think they may not have expected DS to go on past that. When it did, all new characters as if one story that started in 1966 had ended, and another had begun.
What I mean is, I think 1995-1970-1840 is one long storyline revolving around 1840 (and the need to get DS back to a more atmospheric time), a tactic to deal with difficulty in getting fresh ideas, and the inevitable end to the show. [/spoiler]